d663: 11730 - Number Transformation
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Content

You are given an integer number S. You can transform any integer number A to another integer number B by adding x to A. This x is an integer number which is a prime factor of A (Please note that 1 and A are not being considered as a factor of A). Now, your task is to find the minimum number of transformations required to transform S to another integer number T.

EX: 6 12

6->9->12   2次

6->8-> 10->12 3次

Input

For each test case, there will be a line with two integers, S (1<=S<=100) & T (1<=T<=1000), as described above. The last test case will be followed by a line with two 0 s denoting end of output. This case should not be processed.

Output

For every test case except the last one, print a line of the form “Case X: Y” where X is the serial number of output (starting from 1). Y is the minimum number of transformations required to transform S to T. If it is not possible to make T from S with the given rules, Y shall be -1.

Sample Input #1
6 12
6 13
0 0
Sample Output #1
Case 1: 2
Case 2: -1

Hint ：
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